This invention relates to a color television picture tube with an improved shadow mask mounting to minimize misalignments caused by thermal deformations and/or shock vibrations.
FIG. 1 shows a conventional color picture tube having an apertured shadow mask 2 to implement color selection or separation in the usual manner. The mask is mounted in a support frame 3, which is in turn mounted to the inner edges or skirt of a phosphor screen 1 by spring arms 4 attached to panel pins 5 extending inwardly from the screen.
The distance Q (FIG. 4) between the shadow mask 2 and the inner surface of the screen 1 is determined by: EQU Q=(L.times.P)/(3.times.S),
wherein L is the distance between the electron gun and the screen, S is the interval between the electron beams emitted from the gun, and P is the pitch or spacing between the shadow mask apertures. As may be seen from the equation, Q increases as L and P become larger and as S becomes smaller, which follows the current industry trend towards larger color picture tubes and smaller electron guns.
As a result of these large Q distances between the screen 1 and the shadow mask 2, the support frame 3 often projects beyond the skirt 11 of the screen as shown in FIGS. 2 and 4. Such projection creates a particular problem during the process of assembling the screen and the shadow mask; during assembly the screen faces upwardly as seen in FIG. 4 and the support frame thus receives the full weight of the screen. This leads to the deformation of the frame and thus the shadow mask, which in turn causes misalignment between the mask apertures and the phosphor dots or stripes on the screen.
In an effort to solve this problem the construction shown in FIG. 3 has been proposed, wherein the edge of the shadow mask, beginning at point D, is bent to form an unapertured, outwardly extending flange or step portion 22 perpendicular to the tube axis Z. Such a construction is termed a Hip Up configuration, and while it avoids the support frame projection problem, it renders the shadow mask unduly susceptible to Z axis deviations due to thermal deformation as shown in FIG. 5(A) and mechanical shock as shown in FIG. 6(A). Such deviations result from the low degree of stiffness in the vicinity of the step portion 22 and the skirt 21 of the shadow mask, which again leads to misalignment between the mask apertures and the screen phosphors.